The effects of stressors on the immune system are particularly relevant for older adults: Potent stressors such as bereavement become more common with age, immune reactions to stress grow larger with age, and vulnerability to immunological dysregulation and immunologically mediated disease increases with age. Previous studies have linked stressors in old age with increased distress and physiological dysregulation, including depression, high levels of cortisol, poor responses to vaccination, and high levels of the proinflammatory cytokine IL-6. The proposed research will address limitations of the extant literature by focusing on the prospective effects of stressors among older adults and by including individual differences in repetitive thought (RT), that is, the cognitive interpretation and processing of stressful events. Individual differences in RT can create vulnerability or resilience to stressors: Negative, uncontrolled, and intrusive thought (e.g., worry) can prolong and amplify negative consequences, whereas positive and controlled thought (e.g., reflection) can buffer against negative consequences and promote positive consequences such as growth. In this 5-year longitudinal, prospective study, married adults aged 60 or older (N = 183) will be interviewed and will collect saliva samples for cortisol every 6 months;they will be vaccinated annually against influenza, and serum for antibody and IL-6 will be collected before and after vaccination and at the end of influenza season. The primary aim of the study is to test the following predictions: (1) stressors and RT predict psychological and physiological indices of health or dysregulation;(2) RT moderates the effects of stressors;(3) psychological indices (e.g., depression) mediate the relationship between stressors and RT and physiology. A secondary aim of the study is to relate cortisol, autonomic activity and inhibition, IL-6, and antibody to each other and to evidence of influenza infection. Another secondary aim is to validate the dimensional structure of RT in older adults and link trait RT (e.g., trait worry) to episodic RT (e.g., thoughts about a specific stressor). Relevance to public health: Stress in older adults can increase their vulnerability to both mental and physical illness. This research will test whether thought styles such as worry put older adults at greater risk. This knowledge could be used to identify and intervene with the most at-risk individuals.